1. Field of the Invention
The present invention relates to a substrate for a liquid crystal display that constitutes a display of an information apparatus and a liquid crystal display having the same.
2. Description of the Related Art
A liquid crystal display is constituted of a TFT substrate on which a thin film transistor (TFT) is formed in each pixel region, an opposite substrate provided opposite to the TFT substrate, and a liquid crystal sealed between the substrates. A TFT substrate for a transmissive liquid crystal display having inverted staggered type TFTs with a channel protection film is manufactured using five photo-masks, in general. A first photo-mask is used for forming gate bus lines, and a second photo-mask is used for forming a channel protection film. A third photo-mask is used for drain bus lines and source and drain electrodes, a fourth photo-mask is used for forming contact holes by providing openings in the protection film on the source electrodes. A fifth photo-mask is used for forming pixel electrodes. It is advantageous to reduce the number of photo-masks in reducing the manufacturing cost of a TFT substrate.
Unlike a transmissive liquid crystal display manufactured using five photo-masks, in the case of a TFT substrate for a reflective liquid crystal display having TFTs in the same configuration and having pixel electrodes that also serve as light reflecting plates, one photo-mask can be deleted because drain bus lines, source and drain electrodes, and pixel electrodes (reflective electrodes) are simultaneously formed from the same material. Further, since the drain bus lines, source and drain electrodes, and pixel electrodes are formed in the same layer, there is no need for a protective film between the source/drain electrodes and the pixel electrodes which is dispensable in a transmissive liquid crystal display.
In a reflective liquid crystal display, in general, alignment films are directly formed on a top surface of drain bus lines on a TFT substrate and on a top surface of a common electrode on an opposite substrate. If a conductive foreign substance enters the gap between the two substrates when they are combined, the foreign substance can penetrate the alignment films to contact a drain bus line and the common electrode directly. This results in short circuiting between the drain bus line and the common electrode. When short circuiting occurs between the drain bus lines and the common electrode, a line defect occurs because a grayscale voltage is not applied to the pixels on the same drain bus line. Since a line defect is a fatal detect of a product, it can reduce the yield of manufacture of liquid crystal displays.
In order to prevent a line defect attributable to short circuiting as described above, a protective film may be formed on drain bus line. However, when a protective film is formed on the drain bus lines, the protective film will extend on to regions which must be exposed such as regions above pixel electrodes and regions above drain bus line terminals that must be conductive. This results in a need for using a separate photo-mask to pattern the protective film such that openings are formed above the pixel electrodes and drain bus line terminals, which leads to problems such as an increase in the number of manufacturing steps and an increase in the manufacturing cost.